1. Field of the Invention
The present invention relates to a vertical cavity surface-emitting laser.
2. Related Background Art
A vertical cavity surface emitting laser (VCSEL) is one type of the surface-emitting lasers.
This laser is promising in the fields of communication, electrophotography, sensing, and so forth owing to ease of integration and arraying, high efficiency of coupling with an external optical system, and a low production cost. This laser has been already commercialized in a communication field such as infrared short distance communication.
Laser devices are being investigated which employ a thin grating structure in place of a distributed Bragg reflector (DBR) as the reflector of the surface emitting laser.    (Patent Document 1) U.S. Pat. No. 6,055,262    (Patent Document 2) Japanese Patent Application Laid-Open No. 2006-073823
Patent Document 1 discloses a photonic crystal surface-emitting laser employing as the mirror a structure which utilizes a guided resonance (GR) phenomenon by which the light beam introduced vertically onto the surface of a one-dimensional photonic crystal can be reflected at a ratio of nearly 100%.
The mirror of a photonic crystal utilizing the GR phenomenon enables constitution of a surface emitting laser thinner than the surface-emitting laser employing the DBR.
Specifically, a surface-emitting laser can be constituted by employing a grating mirror of several hundred micrometers thick in place of a DBR of several micrometers thick.
In the vertical cavity surface-emitting laser, the control of the lateral mode oscillation is important. In application to communication, the lateral mode output should be of a single mode.
To obtain the single lateral mode, in the conventional vertical cavity surface-emitting laser, a current-confining structure is formed by selective oxidation in the device to restrict the light-emission region of the active layer and simultaneously a waveguide structure is formed in the selectively oxidized portion.
By such a method, however, the oxide aperture should be made smaller for the single lateral mode oscillation. The smaller diameter narrowed by oxidation decreases the light emission region, making difficult a high output of the laser.
Therefore, methods are disclosed in which the reflectivity at the center portion of the mirror of the resonator is made higher than the peripheral portion thereof to facilitate oscillation in a single lateral mode as the fundamental mode.
Patent Document 2 discloses a surface emitting laser in which the reflectivity of the DBR is made higher at the center portion than the peripheral portion by constituting a DBR mirror by alternate lamination of pairs of two kinds of semiconductor films which have refractive index differences between the paired films increasing with the distance from the active layer.